In a chromatograph/mass spectrometer such as GC/MS or LC/MS, the various components contained in a sample are separated in the time direction by a chromatograph, and the ions derived from the separated components are detected. To perform quantitative analysis of a target compound in a sample by GC/MS or LC/MS, generally, a mass chromatogram (also called an ion extraction chromatogram) at a mass-to-charge ratio corresponding to the target compound is prepared, and the area value of chromatogram peaks appearing near the retention time of the target compound in the mass chromatogram is determined. Those area values are then compared to a calibration curve (a regression curve associating concentration with area) prepared in advance, and the concentration i.e. quantitative value of the target compound is computed. Therefore, to increase the precision of quantitative analysis, it is necessary to prevent overlap of components other than the target compound, i.e. of impurity components, with the chromatogram peak on the mass chromatogram.
Thus, conventionally, for example, physical or chemical pretreatment would be performed on a sample to remove impurity components prior to analysis to the extent possible, or the separation conditions of the chromatograph would designed so as to avoid overlap between the target compound and impurity components to the extent possible. However, in cases where there are numerous impurity components or where unknown impurity components are mixed in, by the techniques described above, it is difficult to completely avoid overlap with impurity components that would hinder quantitative analysis.
As another technique for increasing the precision of quantitation of a target compound, changing the mass-to-charge ratio of the mass chromatogram for performing quantitative computations may be considered. Namely, there are usually not one but multiple peaks (mass spectrum peaks) observed on the mass spectrum for a given target compound. It is unlikely that the same effect of overlap of impurity components will exist for all the mass-to-charge ratios at which those multiple peaks appear, and since there will be mass-to-charge ratios with little or no overlap of impurity components, by suitably specifying the mass-to-charge ratio for generating the mass chromatogram for quantitative analysis (hereinafter referred to as “quantitation mass-to-charge ratio”), it is possible to reduce the effect of impurity components and improve quantitative characteristics.
Furthermore, if impurity components with a peak appearing at the aforementioned quantitation mass-to-charge ratio on the mass spectrum are present, it is difficult to distinguish the target compound from the impurity components based on the quantitation mass-to-charge ratio alone. Thus, generally, a confirmation mass-to-charge ratio is specified separately from the quantitation mass-to-charge ratio, the relative ratio (hereinafter referred to as “confirmation ion ratio”) between the peak intensity for the confirmation mass-to-charge ratio and the peak intensity for the quantitation mass-to-charge ratio is determined on a mass spectrum representative of the peaks appearing on the mass chromatogram at the quantitation mass-to-charge ratio, and if the confirmation ion ratio is within a predetermined range, the peak of that mass chromatogram is judged to be derived from the target compound. For this purpose, the confirmation mass-to-charge ratio is also selected to be a mass-to-charge ratio with as little overlap as possible with impurity components other than the target compound.
With a conventional GC/MS or LC/MS data processing device, the analyst can determine and set the aforementioned quantitation mass-to-charge ratio and confirmation mass-to-charge ratio as one of the measurement parameters. To this end, the analyst visually checks the standard mass spectrum of a known target compound, and selects, through trial and error, from among the mass-to-charge ratios for which a clear peak can be observed on the mass spectrum, a mass-to-charge ratio such that the shape of chromatogram peak on the mass chromatogram is a shape close to normal distribution.
However, even when the target compound is the same, if the other impurity components contained in the sample are different, the quantitation mass-to-charge ratio and confirmation mass-to-charge ratio will need to be modified in some cases, so particularly in cases where there are many target compound types, the work of determining the quantitation mass-to-charge ratio and confirmation mass-to-charge ratio is very laborious and takes much time. Furthermore, in cases where the target compound and impurity components overlap completely, operations which depend on the analyst's experience and skill becomes necessary, e.g. comparing mass chromatograms at multiple mass-to-charge ratios and finding a mass-to-charge ratio with the least influence of impurity components, so it is possible that differences in analyst experience and skill will be reflected in the analysis results.
In Patent Literature 1 and Non-patent Literature 1, the present applicant proposed an algorithm (hereinafter referred to as “time series minimum point plotting method”) for accurately estimating the shape of the chromatogram peaks of a target compound even when complete component separation with a chromatograph is not possible and the target compound is mixed with unknown impurity components. The basic idea of this method is as follows.
When a target compound is present in a sample, a peak derived from that target compound will appear near the retention time of the target compound on a mass chromatogram at a specific mass-to-charge ratio. If other components are not present around this retention time, it should be possible to represent the mass spectrum at various times near the retention time of the target compound using a constant scaling factor of the standard mass spectrum of the target compound. By contrast, if impurity components are present near the retention time of the target compound, the peak intensity of the measured mass spectrum will increase on account of the impurity components. However, normally, since many peaks are present in the standard mass spectrum of the target compound, it is unlikely that impurity components will affect all of the many peaks in the measured mass spectrum at a given time. Therefore, it can be surmised that, in the mass spectrum for each time, a peak derived from the target compound and not affected by impurity components will appear at least at some of the mass-to-charge ratios. Namely, by determining the intensity ratio of peaks at a mass-to-charge ratio not affected by impurity components at various times near the retention time of the target compound, it is possible to estimate the chromatogram peak shape of the target compound from which the influence of impurity components has been eliminated.
In Patent Literature 1 and Non-patent Literature 1, the similarity between chromatogram peaks estimated as described above and calculated chromatogram peaks is evaluated to determine if the target compound is contained in a sample. In the case of such compound identification, even if the estimated chromatogram peaks contain some impurity components, the identification precision will rarely be reduced significantly. By contrast, when performing quantitation of a known compound, if impurity components overlap the chromatogram peak serving as the basis for the quantitation, that overlap will directly lead to a decrease in quantitation precision. Therefore, for high precision quantitative analysis, a time series minimum point plotting method as described above is inadequate.